(a) Field of the Invention
The present invention relates to method of regulating mammalian target-of-rapamycin (mTOR) based on a novel finding of a mechanism of regulating mTOR by a phospholipase D (PLD), and a Ras homolog enriched in brain (Rheb). Further, the present invention also relates to a method of screening inhibitors of mTOR, and a method and a composition for treating mTOR-related metabolic diseases by inhibiting mTOR.
(b) Description of the Related Art
mTOR is a serine/threonine protein kinase and a member of a novel superfamily of signaling proteins termed PI 3-kinase related kinases (PIKKs), based on sequence similarity of their catalytic domains. The mTOR pathway is an emerging target for the treatment of cancer, diabetes, obesity, hamartoma syndromes, tissue/organ hypertrophy, etc. Recent studies have demonstrated its role as a mediator of lifespan control in C. elegans and Drosophila. Despite the significance of this pathway in such diverse biological processes, the mechanism of its regulation by nutrients remains unknown.
In addition, mTOR requires the lipid second messenger phosphatidic acid (PA) for its activation. PA is an enzymatic product of PLD. PLD, which hydrolyzes phosphatidylcholine (PC) to generate PA, constitutes another branch of the mTOR upstream regulators through which mitogenic signals impinge on the mTOR pathway. Mammalian PLD isozymes identified to date, PLD1 and PLD2, sense a variety of signals, such as neurotransmitters, hormones and growth factors, to regulate multiple physiological events such as proliferation, secretion, respiratory burst and actin cytoskeletal reorganization, and the like.
PA binds to mTOR and activates its activity to phosphorylate S6K1 and 4EBP1, two known downstream effectors of mTOR19. However, the mechanism by which PA activates mTOR in cells remains unknown. Recently, the inventors found that PLD2 specifically forms a functional complex with the mTOR/raptor complex to transducer mitogenic signals, and suggested that the localized generation of PA is essential for PLD2 activation of mTOR kinase activity as provided by the interaction with raptor through TOS motif in PLD2 (Ha, S. H., Kim, D. H., Kim, I. S., Kim, J. H., Lee, M. N., Lee, H. J., Kim, J. H., Jang, S. K., Suh, P. G., Ryu, S. H. PLD2 forms a functional complex with mTOR/raptor to transduce mitogenic signals. Cell. Signal. 18 (2006) 2283-2291, which is hereby incorporated by reference). The above identification of PLD2 as a functional and physical mediator for the mTOR/raptor complex led the inventors to test the interrelations between PLD2 and Rheb, to complete the present invention.